Heat accumulator

ABSTRACT

A heat accumulator which comprises a eutectic mixture of LiF and fluoride having a melting point of less than 850* C of the group constituted by sodium fluoride and/or magnesium fluoride. The mixture may additionally comprise potassium fluoride and/or calcium fluoride. The heat accumulator is suitable for supplying thermal energy to a hot-gas engine and for use in heat accumulating ovens.

United States Patent m1 Schroder 51 Jan. 9, 1973 HEAT ACCUMULATOR [56]References Cited [75] Inventor: Johann Schroder, Aachen, Germany UNITEDSTATES PATENTS [73] Assignee: U.S. Philips Corporati n, ew 3,029,5964/1962 l-lanold etal. ..60/24 York, N.Y. 3,400,249 9/1968 Mekjean et al..l26/4 00 3,470,865 10 1969 M k' t l ..l26 400 22 Filed:- Aug. 24, 1971I 6 e a l [21] 'Appl. No.: 174,357 Primary Examiner-Martini. SchwadronAssistant Examiner-l-larold Burks, Sr. Attorney-Frank R. Trifari [30]Foreign Application Priority Data Aug. 29, i970 Netherlands ..7o12s3o 1ABSTRACT June 23, 197] Netherlands ..7i08625 A heat accumuIator whichcomprises a eutectic mix. 7 ture of UP and fluoride having a meltingpoint of less [52] U.S. Cl. ..l26/400, 60/24, 252/71 than 850 C of thegroup constituted by sodium [51] Int. Cl ..F24h 7/00 fluoride and/ormagnesium fluoride. The mixture may [58] Field of Search .....60/ 36,.24; l26/400; 165/104; additionally comprise potassium fluoride and/orcalci- 252/70, 71 um fluoride. The heat accumulator is suitablefor-supplying thermal energy to a hot-gas engine and for use in heataccumulating ovens.

6 Claims, 1 Drawing Figure PATENTEUJAH 9 I973 3, 709,209

I NVE NTOR.

JOHANN SCHRODER BY v.

AGENT HEAT ACCUMULATOR The invention relates to a heat accumulatorcomprising a reservoir which contains an inorganic material suitable foraccumulating heat and means for supplying and conducting away heat. Theinvention particularly relates to heat accumulators having a high heataccumulating output per unit volume and weight, for example, for use asan accumulating oven for room heating or in combination with heat energymachines such as hot-gas engines.

Combinations of hot-gas engines and heat accumulators in which the heataccumulator is in heat exchanging contact with the heater of the hot-gasengine either directly or by means of a heat transporting system areused, for example in those cases where a primary heat source is notcontinuously available or cannot be used. In this case the use of solarenergy may be considered which is only available for a part of the24-hour period. lfthe primary heat source requires the supply of air andthe removal of combustion gases, similar situations may occur. Considerin this case under-water uses and vehicles which are used in areas wherethe removal of combustion gases in the atmosphere is prohibited or issubject to stringent restrictions.

The use of a heat accumulator for the supply of heat to, for example, ahot-gas engine is known per se. It has been proposed to use a heataccumulator for this purpose, which in its simplest form consists of areservoir filled with lithium hydride (melting point 680 C), lithiumhydroxide (melting point 450 C) or lithium fluoride (melting point 848C) (see US. Pat. No. 3,080,706).

As compared with the other compounds mentioned in the patentspecification lithium fluoride has the highest heat content per unitvolume and it is stable at temperatures up to 900 C and is chemicallylittle aggressive when it is anhydrous and when it does not containoxygen in the form of oxides or other compounds.

In accumulating stoves sintered magnesite (MgO) is generally used as aheat accumulating material. The thermal capacity of this material perunit volume and weight 7 is relatively low. As a result, such ovensgenerally have a larger volume than other known heating devices.Accumulating ovens which contain cast iron as a heat accumulatingmaterial are also sometimes used in central heating installations. It istrue that the thermal capacity per unit volume of this material isslightly larger than that of magnesite, but the thermal capacity perunit weight is clearly less than that of magnesite. This means that dueto the large floor load, such stoves can only be accommodatedeconomically in cellars oflarge buildings.

The high price, the limited availability and the relatively high meltingtemperature stand in the way of using lithium fluoride on a large scaleas a material for storing thermal energy. In addition there are onlyfew, very expensive materials which are corrosion resistant for a longperiod at temperatures of more than 800C.

The object of the present invention is to provide a solution to theseproblems.

It was found that the envisaged object may be achieved by a heataccumulator comprising a reservoir which contains an inorganic materialhaving a melting point of less than 850 C and means for supplying andconducting away heat, which is characterized in that the inorganicmaterial mainly consists of a eutectic mixture of lithium fluoride andone or more fluorides chosen from the group constituted by sodiumfluoride, potassium fluoride, calcium fluoride and magnesium fluoride,on the understanding that potassium fluoride as well as calcium fluoridecan only be present in com? bination with sodium fluoride or magnesiumfluoride in addition to lithium fluoride in the eutectic mixture, andthis in quantities of less than 30 mol percent of potassium fluoride andless than 40 mol percent of calcium fluoride.

Eutectic mixtures having larger quantities of potassium fluoride orcalcium fluoride are found to have a too low thermal content to betechnically usable. In the eutectic mixtures which are used according tothe invention potassium fluoride and calcium fluoride serve in the firstplace to get a relatively low eutectic melting point. Suitable eutecticmixtures including potassium fluoride and calcium fluoride containsodium fluoride or magnesium fluoride in addition to lithium fluoride.

The heat of fusion and the thermal capacity of the fluorides of sodium,potassium, calcium and magnesium are relatively high by themselves, butthe high melting temperature stands in the way of using the purefluorides for storing heat in the form of latent melting heat.

By adding a fluoride to lithium fluoride the melting point is of coursedecreased. For use in a heat accumu v lator this is, however, notsufficient. In order to avoid demixing of the melt and deposition of thehigher melting fluorides, particularly at those areas where heat istaken up, a eutectic mixture is to be chosen.

It was surprisingly found during experiments that the thermal expansionduring melting of eutectic mixtures which are used according to theinvention is smaller than was to be expected on the ground ofcalculations based on the thermal expansion during melting of the purecomponents. The volume of the reservoir must of course correspond to thevolume of the heat accumulating material at the highest temperaturewhich is to be allowed during use. The volume of the reservoir may thusbe smaller than would be expected when using the eutectic mixturesaccording to the invention.

The melting point of the eutectic mixtures used accordingto theinvention is generally below 800 C. This means that a larger number ofmaterials is available from which the heat accumulator can be built up.

When using the eutectic mixtures according to the invention, aconsiderable economy is obtained because the said fluorides are marketedat much lower prices than lithium fluoride. The price of a eutecticmixture, for example, lithium fluoride and magnesium fluoride which hasa higher thermal capacity per unit volume than lithium fluoride, is lessthan half the price of lithium fluoride. In these cases where thethermal capacity is smaller than for lithium fluoride, the difference isso small that the emanative drawback is offset by the advantage of thelowerprice, the wider availability and the lower melting temperature.

In the Table below several eutectic mixtures are compared mutually andwith lithium fluoride, sintered magnesite (MgO) and cast iron. The'Tablestates the quantity of heat which can be accumulated or supplied in thetemperature range of from 450 to 860 C.

TABLE 60 mol. percent LiF plus 40 mol. percent NaF.

magneslte (MgO) The Table shows that when using eutectic rnixtures.

at considerably lower temperatures than is the case for lithiumfluoride, a considerable quantity of thermal energy can be stored aslatent heat of fusion. At temperatures below the melting point oflithium fluoride this makes it possible to store a quantity of heatwhich is considerably larger than would be possible with the samequantity of lithium fluoride at the same temperature. This means that ifthe temperature of the molten mixture is not increased too far above themelting point, relatively cheaper materials canbe used for those partswhich come in contact with the melt than would be possible when usinglithium fluoride only.

Furthermore the Table shows that the thermal capacity per unit weightand volume is considerably larger than for sintered magnesiteand castiron. As compared with these materials, the said eutectic mixtures havethe advantage that between 450 and 860 C approximately half theaccumulated heat can be taken up at a constant temperature. The latteris especially important when using heat accumulators according to theinvention in combination with heat energy machines whose output istemperature dependent. By

using the latent melting heat it is possible to take up heat during acomparatively long period at a constant temperature and hence a constantoutput can be obtained.

It is evident that the mixtures need not have an exact eutecticcomposition at the instant when the reservoir of the heat accumulator isfilled with this mixture. A deviation of a few percent (less than, forexample, 2 percent by weight) may generally be tolerated. After severaltimes of heating until the mass has melted and after cooling, the excessof one ofthe components which has a higher melting point than theeutectic composition has deposited and a eutectic mixture is the result.Since this deposition will particularly take place at those areas in thereservoir where heat is taken up, this may result in a poorer transferof heat. The deviations of the eutectic composition must therefore bepreferably as small as is possible in practice. Technical qualities of,for example, 99 percent purity may, however, be used. The presence ofsmall quantities of impurities may result in a small deviation of thepreviously mentioned melting points. it is, however, desirable that thefluorides used be anhydrous so as to avoid corrosive attack of thereservoir and other metal parts with which the fluorides come in contactand do not contain oxygen in the form of oxides or other compounds.Fluorides may be rendered anhydrous and free from oxygen compounds bytreating the fluorides in a molten state with ammonium fluoride orammonium bifluoride until the melt which was initially opalescent clueto the presence of oxygen compounds has become bright.

Thermal content in the temperature range of from 450 C. to 860 0. Heatof fusion Melting point, C. In caL/ml. In caL/gr. In caL/rnl; In caL/gr.

The heat accumulator according to the invention may be used incombination with any type of hot-gas engine. A description of a hot-gasengine provided witha cylinder in which a piston and a cooperatingrepeller define a hot space (expansion space) and a cold space isdescribed in Philips Technical Review 20, pages 245-262 1958/1959. Ifthe heater consists of a system of ducts through which the workingmedium flows on its path to and from the expansion space, these may bein direct heat exchanging contact with the eutectic mixtures in thereservoir of the heat accumulator. If desired the heat may alternativelybe transferred with a heat-transferring medium, for example, a liquidNa-K alloy which is circulated in a system which is at one end in heatexchanging contact with the heat accumulator and at the other end withthe heater of the'hot-gas engine. Also so-called heat pipes may be usedfor this purpose.

In order that the invention may be readily carried into effect it willnow be described in detail by way of example with reference to theaccompanying diagrammatic drawing which comprises a sole FIGURE.

The FIGURE shows a hot-gas engine I, a heat accumulator 2, a burner 3with fuel supply' 12, a preheater 4, a system of pipes 5 and 6 and apump 7 for the supply of air and removal of combustion gases 6,respectively. The air is passed through the preheater 4 in which thecombustion gases give off heatto the air. in additiona system of pipes 8including a pump 9 is provided. The pipes 8 contain a liquid alloy, forexample, a sodiumpotassium alloy. Under those circumstances when com,-bustion gases can or may be removed in the at mosphere, heat is suppliedin the burner 3 to the liquid alloy which is circulated in the system ofpipes 8. Coming from the burner the heated liquid alloy first flowsthrough the heat accumulator 2 and through a number of pipes 8'. Theaccumulator comprises a reservoir 10 filled with one of the saideutectic mixtu res,.for example, 67 mol. percent UP and 33 mol. percentMgF- Part'of the thermal energy is passed on to the eutectic mixture.Subsequently the alloy flows alongthe heater ll of the hotgas engine andvia the pump 9 back to the burner. In the heater 11 part of the heat ispassed on to the hot-gas engine. During the periods when combustiongases cannot or may not be removed. in the atmosphere, the heat storedin the heat accumulator 2is transferred by means of the liquid alloycirculated through the pipes 8 to the heater 11 of the hot-gas engine l.g It is of course alternatively possible tofstore electrical heat in theheat accumulator. To this end electrical heating elements are providedin and/or around the heat accumulator. if desired a burner, for example,for emergency cases may also be present in such a system.

What is claimed is:

l. A heat accumulator comprising a reservoir which contains an inorganicmaterial having a melting point of less than 850 C and means forsupplying and conducting thermal energy, away from said reservoirwherein the inorganic material consists mainly of a eutectic mixture oflithium fluoride and at least one fluoride selected from the groupconsisting of sodium fluoride, potassium fluoride, calcium fluoride andmagnesium fluoride, potassium fluoride or calcium fluoride being presentonly in the presence of both sodium fluoride and lithium fluoride orboth magnesium fluoride and lithium fluoride, the maximum quantity ofpotassium fluoride being less than 30 mol percent and the maximumquantity of calcium fluoride being less than 40 mol percent.

2. A heat accumulator as claimed in claim 1, characterized in that theinorganic material consists of a eutectic mixture of lithium fluorideand sodium fluoride.

3. A heat accumulator as claimed in claim 1, characterized in that theinorganic material consists of a eutectic mixture of lithium fluorideand magnesium fluoride.

4. A heataccumulator as claimed in' claim 1, characterized in that theinorganic material consists of a eutectic mixture of lithium fluoride,sodium fluoride and calcium fluoride.

5. A heat accumulator as claimed in claim 1, characterized in that theinorganic material consists of a eittectic mixture of lithium fluoride,sodium fluoride and magnesium fluoride.

6. A heataccumulator as claimed in claim 1, characterized in that theinorganic material consists of a eu' tectic mixture of lithium fluoride,magnesium fluoride and potassium fluoride.

2. A heat accumulator as claimed in claim 1, characterized in that theinorganic material consists of a eutectic mixture of lithium fluorideand sodium fluoride.
 3. A heat accumulator as claimed in claim 1,characterized in that the inorganic material consists of a eutecticmixture of lithium fluoride and magnesium fluoride.
 4. A heataccumulator as claimed in claim 1, characterized in that the inorganicmaterial consists of a eutectic mixture of lithium fluoride, sodiumfluoride and calcium fluoride.
 5. A heat accumulator as claimed in claim1, characterized in that the inorganic material consists of a eutecticmixture of lithium fluoride, sodium fluoride and magnesium fluoride. 6.A heat accumulator as claimed in claim 1, characterized in that theinorganic material consists of a eutectic mixture of lithium fluoride,magnesium fluoride and potassium fluoride.